1. Field of the Invention
This invention is concerned with curing agents for isocyanato-substituted prepolymers and with the use of these curatives in preparing cured polyurethanes. More specifically, this invention is concerned with aromatic diamino disulfides as curatives in the preparation of crosslinked polyurethane compositions.
2. Description of the Prior Art
Polyurethane elastomers possess excellent physical properties, such as good elasticity, high tensile and tear strengths and excellent resistance to chemical solvents and to abrasion, and their commercial use has been increasing at a rapid rate. These materials are formed by reacting a hydroxy-terminated polymer, such as a polyether or a polyester for example, with a polyfunctional isocyanate and a curing agent. This reaction can be carried out in one step in which all components are mixed together directly, or, more frequently, by a two-step method, generally referred to as the prepolymer technique. In the latter technique the polymeric diol is first reacted with an amount of diisocyanate in excess of the stoichiometric requirement to yield an intermediate polymer bearing reactive isocyanato groups. This polymer is variously referred to in the trade as an isocyanato-terminated prepolymer, or a polyurethane prepolymer or a polyisocyanate prepolymer. Such prepolymers can be readily purchased and are used by a large variety of small and medium-sized formulators together with a curative to form shaped polyurethane articles. Because of its ease and convenience in use, the reproducibility of results, and the savings in time, the prepolymer technique enjoys wide use.
Prepolymers can theoretically react with any compound containing sufficiently labile hydrogens to yield chain-extended or cross-linked polymers. In practice, however, compounds suitable for cross-linking, generally referred to as curatives, curing agents, or hardeners, have been limited to polyols and to primary aromatic diamines, and the precise nature of the specific curing agent has been found to have a significant, yet largely unpredictable effect on the elastomer properties. It has been generally recognized, though, that prepolymers cured with aromatic diamines exhibit superior physical properties and require shorter cure cycles than polyolcured compositions, and that among aromatic diamines those with the amino groups in the para positions are far better curatives than the corresponding isomers in which the amino groups occupy ortho positions.
A great variety of aromatic diamines have been examined as curatives but surprisingly very few have been found suitable for practical use. Most appear to be unsatisfactory for a variety of technical and/or economic reasons, such as lack of ready availability, too high a cost, too high a melting point, too great a reactivity, too short a pot life, too high a toxicity, limited compatibility with prepolymers, poor physical properties of the cured product, and so forth. Summaries of results obtained in studies with diamine curatives can be found, for example, in J. H. Saunders and K. C. Frisch "Polyurethanes, Chemistry and Technology, Part I, Chemistry", Interscience Publishers, New York, N. Y. 1962, pages 279 and 295, and in L. N. Phillips and D. B. V. Parker "Polyurethanes, Chemistry, Technology and Properties" Illife Books Ltd. London, 1964, page 46. Another reference which reports on aromatic diamine curatives is S. Hashimoto, T. Koide, and J. Sunamoto, Chemistry of High Polymers, Tokyo, 22(7), 422-428 (1965). The latter reference includes results obtained with standard aromatic diamines and also with specific aromatic diamines derived from anilines by coupling them in the para position through the bridging groups, --CH.sub.2 --, --CH=CH--, --S--S--, --SO.sub.2 --, and --CO--. Again, some of these bridged diamines react too rapidly and possess pot lives too short for practical use. Others, such as sulfonyl bisaniline proved to be too high melting, and 4,4'-diaminodiphenyldisulfide was found to be too reactive, resulting in a pot life shorter than that commercially acceptable and to cure poorly resulting in cured products which were distinctly soft and inferior in physical properties.
4,4'-Methylene bis(2-chloroaniline) generally known and referred to as MOCA yields cured products with excellent properties and among aromatic diamines has gained nearly universal preference for commerical use, despite the fact that its compatibility with prepolymers is often poor, that its melting point necessitates a relatively high working temperature, that it frequently exhibits an unsatisfactorily short pot life, and most importantly is held to be toxic. Another aromatic diamine which has also found some commercial use in some instances is 3,3'-dichlorobenzidine. In 1973, (Federal Federal Register, Vol. 38, Page 10929 May 3, 1973), the widespread use of MOCA was dealt a very severe blow by an emergency temporary standard established by the Department of Labor under the Williams-Steiger Occupational Safety and Health Act (OSHA) of 1970. This standard places MOCA on a list of alleged carcinogens and sets forth very restrictive and costly work practices and stringent, expensive controls and safeguards designed to protect personnel from exposure to these compounds. 3,3'-Dichlorobenzidine is also included in the same standard.
Because of the lack of an available substitute curative these OSHA standards and regulations threaten to have a grave impact on the cast polyurethane elastomer industry, especially on the many small and medium-sized molders who are unable to meet the expense necessaray to comply with the measures required by law to handle MOCA. As a consequence a feverish research effort has ensued to develop other, safer, yet effective, curatives for isocyanato-terminated prepolymers.